I. Field of the Invention
This invention relates to an apparatus and method for deforming a workpiece. In particular, the apparatus and method of the present invention is suitable for regulating the temperature of one or more dies and a workpiece placed between the dies before, during and after deformation of the workpiece.
II. Description of the Relevant Art
In the manufacture of various workpieces, it is necessary to shape those workpieces by such methods as casting, machining, consolidating smaller pieces (i.e., welding) and deformation. Metal deformation is a process which exploits a remarkable property of metals--their ability to flow plastically in a solid state without concurrent deterioration of properties. Thus, a workpiece may be reshaped or deformed without losing its inherent strength, etc.
Deformation processes usually involve heating the workpiece prior and/or during a shaping or compression operation. However, cold-deformation can also be used. If the workpiece is heated above its recrystallization temperature, the workpiece deformation process is generally referred to as "hot working" the workpiece. Hot working is advantageous in that grain distortion is minimized during the deformation operation.
There are numerous hot working processes, several of which are rolling, forging, extrusion, pinch and roll, etc.
Rolling is usually the first step in converting a workpiece into sheets, plates, bars and strips of finished product. Basic rolling consists of passing a heated workpiece between two rolls that revolve in opposite directions. The space between the rolls is somewhat less than the thickness of the entering workpiece. Thus, the metal is squeezed and elongated, and usually changed in cross section. It is very important that the entering workpiece be heated uniformly throughout its cross-section. This usually requires prolonged heating at the desired temperature, a process known as soaking. Gas- or oil-fired soaking pits are often used to pre-heat the workpiece prior to it entering between the two rolls.
Forging is a thermomechanical process for shaping a solid workpiece by the application of force, either impact or continuous, to create a different geometry from that of the original workpiece or billet geometry. As used herein, forging includes open-die forging, closed die forging, swaging, heading, etc. Similar to rolling, the workpiece to be forged is usually heated to an elevated temperature at which the workpiece is malleable and/or ductile. The workpiece may be heated prior to forging, either in a fuel-fired furnace, indirect electric furnace, by electric induction, or direct electric resistance. Thus, like rolling, conventional forging devices generally utilize one of these four methods of heating the workpiece prior to placing a workpiece into a separate compression apparatus. Heating the workpiece prior to forging serves to lower the required force, increase the amount of deformation per compression, and control the final structure (the mechanical properties, the acoustical properties and the homogeneity) of the resulting forged part.
The extrusion process generally involves heating the workpiece and compressively forcing workpiece flow through a suitably placed die to form a product with reduced cross section. By compression upon one or more sides of the workpiece, portions of the workpiece are extruded as flash, for example, through an opening in one of the dies or the chamber surrounding the workpiece.
It is important to note that whatever deformation processes are chosen, whether it be rolling, forging, extrusion, etc., conventional hot-working processes generally involve preheating the workpiece by a separate apparatus from that of the device which performs the shaping or compression operation. Moreover, preheating may be performed in a completely separate chamber from the apparatus which performs the shaping/compression.
Commonly used pre-heating techniques are fairly simple to incorporate. Fuel-fired and electric furnaces consist of a thermally insulated box furnace having an elongated opening or slot for easy insertion and removal of the workpiece. A room-temperature workpiece can be inserted into the furnace and subsequently withdrawn at the forging temperature. The furnace is heated using either fossil fuels or electrically heated coils. Electric induction heating comprises induction coils powered at various current frequencies and amplitudes. Induced current within the workpiece causes heating of the workpiece at a current penetration depth which is a function of input power frequency and amplitude as well as workpiece conductivity and magnetic permeability. Direct resistance heating often utilizes a stepdown transformer with its secondary electrically connected across the workpiece. The transformer generally produces high alternating current which, through resistance of the workpiece, heats the workpiece to the desired temperature. A basic problem with resistance heating using stepdown transformers is that the alternating current from the transformer does not produce even heating throughout the bulk workpiece. In addition, such transformers are limited by practical considerations to output currents of 100,000 amps or less. This, in turn, limits the heating rate that may be achieved. Limited heating rates mean that all processes described above must be done "off-line" separate from the forging process.
Furnace heating, induction heating and resistance heating fed by stepdown transformers all are further limited by what is known as "skin effects." Skin effect generally results in an uneven temperature gradient across a cross-section of the workpiece. The above heating methods generally heat from the surface inward, whereby the temperature at the outer surface of the workpiece is substantially higher than that of the internal portion. In an effort to obviate these problems, homopolar generators can be used as a current source for direct current resistance heating of the workpiece. Homopolar generators provide a direct current source pulse which is unidirectional to allow resistive heating simultaneously throughout the workpiece cross-section, rather than from the surface inward. Keith, et al., "Electrical Heating of Forging Billets," Electric Power Research Institute, Project 1201-18, Final Report (November 1982); Aanstoos, et al., "Heating and Forging Billets Using the Pulsed Homopolar Generator," Center for Electromechanics The University of Texas at Austin: Final Report (Oct. 31, 1982).
Although resistive heating prior to compression or open-loop heating via homopolar generators produces a more uniform temperature gradient throughout the heated workpiece, open-loop resistive heating alone cannot provide strict temperature control during the forging process. Generally speaking, a conventional deformation process comprises a heating mechanism that is separate and distinct from the mechanism which performs compression shaping/compression. Thus, conventional deformation requires that a workpiece be heated to its recrystallization temperature in a heating mechanism such as a furnace or between conductive electrodes of a homopolar generator. Then, after the workpiece is heated, it is removed from the heating mechanism and placed between a pair of dies or die blocks. During the time in which the workpiece is removed from the heating mechanism and then placed between the dies, a considerable amount of heat may be lost from the surface of the workpiece. Furthermore, once the workpiece is in contact with the colder dies, it cools even more rapidly. Thus, once compression pressure is actually applied, workpiece temperature is unknown or workpiece temperature becomes nonuniform. Also, as the heated metal workpiece is compressed between dies, contact between the workpiece and die increases and heat is conducted away from the workpiece, primarily through contact with the die at a higher rate.
Temperature nonuniformity and out-migration of heat become particularly severe when high strength metal alloys and super alloys, such as titanium and nickel-based alloys, are being deformed. These temperature sensitive alloys are only deformable over narrow ranges of temperature and strain rate. Furthermore, conduction of the heat away from the surface of temperature sensitive alloys results in reduced plasticity of the surface material which in turn leads to surface fractures.
Traditional solutions to the problems created by out-migration of heat include: multiple reheating and compression cycles, hot-die working, isothermal deformation, and thermal insulation wrappings. Multiple heating and compression cycles increase microstructural grain size and lead to large in-process inventories. In hot-die working, the dies are typically preheated via conventional methods such as embedded electrical resistance heaters, electrical induction heaters or gas-fired heaters to some temperature above ambient, but below the workpiece deformation temperature, and the hot dies are then placed on the workpiece, whereby deformation then occurs. Warming the die reduces the loss of heat from the workpiece, but does not eliminate it and generally complicates the die and deformation apparatus. In isothermal workpiece deformation, the dies are heated to the same temperature as the workpiece, virtually eliminating conduction of heat away from the surface during the compression operation. However, in order to maintain the necessary die strength at elevated temperature, dies must be made from expensive alloys and the entire deformation mechanism must generally operate in an inert atmosphere, or vacuum, in order to prevent excessive oxidation of the die materials. Accordingly, isothermal deformation, to an even greater extent than hot-die working, requires a complicated mechanism which reduces the throughput of reshaped material.